Armor began to fall out of favor in the 16th and 17th centuries yes and the primary reason are guns. Thats the answer to your question: musketry. Ill keep it brief as /u/DonaldFDraper can hit the particulars a bit more than me but the Snapple Facts are:

By the 30 Years War guns and pikes were, in general, kept in a 1:1 ratio. As guns could reliably penetrate armor along with the price of armor being so high its use began to decrease. Im the English Civil War for instance the British would change to their infamous cloth red coats. Because of this change their armies could move from 10 miles a day to 16.

That's really the big thing. Guns were standardized and cheap. They allowed army sizes to balloon because of ease of training compared to pikes or bows along with standardized production and parts compared to say an English longbow which took up to 7 years to make. Meanwhile armor which was hardly reliable against a gun along with being stupidly expensive to make and highly personalized to the weater was just not useful.

When armies go from a few ten thousand to 200-400 thousand potentially the needs change. When guns completely took over with the rise of a standardized flintlock replacing the burdensome matchlock and ring bayonets the needs of armor and pikes disappeared. Guns were universal and army sizes were enormous. Mobility and standardization was taken over craftsmanship and specialization when the latter became trumped by the former.

The Rise of Modern Warfare: 1618-1815, H.W. Koch

hi! It seemed to me there have been many questions on the declining importance of armour, so I've run a big search on the sub for you; hopefully there will be something useful for your paper in here

why

• When did armies make the switch from armor to uniforms?

• What caused the decline in the use of personal armour?

• Did the use of armored knights fall out of practice immediately with the introduction of firearms in battle?

• Why did 19th century cavalrymen not wear plate armor?

• When did soldiers stop using plate armor?

• How was armor phased out of European warfare?

• Why did armour seem to fall out of use long before the use of melee weapons did?

• Were there any significant battles or other events that demonstrated the obsolescence of armor against firearms?

• Why did soldiers stop wearing armor in the switch from sword combat to gun combat?

• Why did metal armor disappear in Europe?

• Why has decorative armor stopped being used in battle?

when-ish / examples of post-17th c use of armour

• In what year and battle did the last recorded charge of knights in heavy armor occur? What conflict last saw widespread use of knights in heavy armor?

• What was it that ended the traditional knight's dominance on the European battlefield?

• How did metal armor go out of usage?

• How effective were body armors in WWI?

• Did WW1 and WW2 infantry soldier wear any sort of body armor?

• When did modern infantry begin wearing body armor?

• When did armor manufacturers begin to focus on making armor bulletproof?

sources

• Is there a book or set of books out there that accurately depict the way warfare has developed over the centuries? I'm talking about from the time people were using spears, swords, and chariots to now where we use guns, armored vehicles, and drones.

.. and of course, there was the development of armoured vehicles like tanks

Development of the bayonet for the musket allowed the removal of pikes. This led to the all-gun army, and much less emphasis on hand-to-hand combat and edged weapons. Armour was still worth buying and wearing when infantry fought in a pike-musket formation, as less firepower was delivered (fewer guys with guns, less space for manipulating them, much more chance to 'get in close'). Bayonets allowed better defence against cavalry and being rushed by infantry (doubling the number of 'spikes' in a 50-50 formation). They came in around 1700 and by the Seven Years' War armour was all but gone, remaining vestigial in some units such as French Cuirassiers. Swords lasted a bit longer but were also subsequently largely for ornamental purposes, at least among foot soldiers.